It is well known that limiting the exposure of oxygen-sensitive products to oxygen maintains and enhances the quality and "shelf-life" of the product. For instance, by limiting the oxygen exposure of oxygen sensitive food products in a packaging system, the quality of the food product is maintained, and food spoilage is avoided. In addition such packaging also keeps the product in inventory longer, thereby reducing costs incurred from waste and having to restock. In the food packaging industry, several means for limiting oxygen exposure have already been developed. At present, the more commonly used means include modified atmosphere packaging (MAP), vacuum packaging and oxygen barrier film packaging. In the first two instances, reduced oxygen environments are employed in the packaging, while in the latter instance, oxygen is physically prevented from entering the packaging environment.
Another, more recent, means for limiting oxygen exposure involves incorporating an oxygen scavenger into the packaging structure. Incorporation of a scavenger in the package can provide a uniform scavenging effect throughout the package. In addition, such incorporation can provide a means of intercepting and scavenging oxygen as it is passing through the walls of the package (herein referred to as an "active oxygen barrier"), thereby maintaining the lowest possible oxygen level throughout the package.
One example of an oxygen scavenger incorporated into an oxygen scavenging wall is illustrated in European Applications 301,719 and 380,319 as well as in PCT 90/00578 and 90/00504. See also U.S. Pat. No. 5,021,515. The oxygen scavenger disclosed in these patent applications comprises a transition metal catalyst and polyamide. Through catalyzed scavenging by the polyamide, the package wall regulates the amount of oxygen which reaches the interior of the package (active oxygen barrier). However, it has been found that the onset of useful oxygen scavenging by this wall, i.e. up to about 5 cubic centimeters (cc) oxygen per square meter per day at ambient Conditions, may not occur for as long as 30 days. See Example 10 herein. The delay before the onset of useful oxygen scavenging is hereinafter referred to as the induction period.
In fact, other oxygen scavengers and walls which incorporate these scavengers may also exhibit an induction period. For instance oxygen scavengers comprising a transition metal catalyst and an ethylenically unsaturated compound, e.g. polybutadiene, polyisoprene, dehydrated castor oil, etc., may exhibit induction periods. These scavengers are disclosed in copending Ser. No. 679,419, filed on Apr. 2, 1991 for "Compositions, Articles and Methods for Scavenging Oxygen". When the oxygen scavenger comprises a polybutadiene, the induction period can exceed thirty days. Scavengers comprising polyisoprene or dehydrated castor oil typically have shorter induction periods, i.e. about fourteen- and one-day delays respectively. As is evident, the duration of induction periods depends on several factors, some of which are not completely understood or controlled. Accordingly, when using oxygen scavengers having longer induction periods, it will be required to keep the scavenger or oxygen scavenging layers and articles prepared therefrom in inventory prior to using them as scavengers in order to provide reliable scavenging behavior. On the other hand, when using scavengers having shorter induction periods, the layers and articles prepared therefrom will have to be prepared quickly and put to use in a short time period, sometimes immediately, in order to get the maximum effectiveness as a scavenger. Otherwise, they would have to be stored in an oxygen-free atmosphere which can be costly.
One method which could be employed to initiate scavenging on demand in packages for oxygen-sensitive foods involves incorporating photooxidizable rubber, i.e. cis-1,4-polyisoprene, and a photosensitizing dye into the inner surface of a package and then exposing it to visible light. See Rooney, M. L., "Oxygen Scavenging: A Novel Use of Rubber Photo-oxidation", Chemistry and Industry, Mar. 20, 1982, pp. 197-198. However, while this method initiates oxygen scavenging within minutes, and thus allows one to initiate oxygen scavenging when desired, it requires constant exposure of the package to light to maintain the scavenging effect. Even further, because a dye is involved, it would be difficult to employ this method for applications which require colorless packaging, especially the transparent and usually colorless packaging used with food and beverage products.